1. Field of the Invention
The present invention relates to a heat roller fixing device for use in a full-color image generating apparatus. It also relates to an image fixing method, and more particularly to an image fixing method for a full-color electrophotographic copying machine including a heating roller.
2. Discussion of the Background
There is known a device for fixing toner images produced by superposing three or four color toners on a toner carrier (hereinafter referred to as a "copy paper") in a full-color copying machine or the like, the device including a nip region having a heating roller and a pressing roller for heating and pressing the copy paper to fix the toner images. Generally, full-color copies which are glossier than monochromatic copies are preferred, and full-color copies are desired which carry images of high light transmittance (color reproducibility) so that they can be used as film images for use with an overhead projector (hereinafter referred to as an "OHP film image"). For these reasons, toners for use on full-color copies have low molecular weights, and narrow molecular weight distributions or, stated otherwise, low toner cohesion.
If a heating roller having an offset prevention layer of Teflon.RTM. were employed for fixing a full-color copy image which uses toners of low molecular weights and low toner cohesion, hot offsetting and fixing failure would simultaneously be brought about. Heretofore, any fixing device including a heating roller having a surface layer of Teflon.RTM. has not been available for fixing full-color copy images. Conventional fixing devices in full-color copying machines or the like have employed heating rollers made of silicone rubber having good release capability.
Fixing of toner to a toner sheet is regarded as adhesion of the toner to the toner sheet (see, for example, a paper entitled "Fixing ability of electrophotographic toner" at the 51st research meeting of the Electrophotographic Society.)
The paper referred to above indicates that adhesion includes the elemental processes of liquefaction, flowing, wetting, and solidification, and that in order to fix toner, the viscoelasticity of toner is required to be within a certain range due to heat fusion.
In addition, the hot offset of toner (which means toner transfer when the toner is heated) is generated by cohesion destruction which occurs when the adhesive force between the toner and the heating roller is stronger than the cohesive force of the toner.
Therefore, the toner viscoelasticity and adhesion are important parameters for fixing toner.
The toner viscoelasticity is affected by the molecular weight and molecular weight distribution of toner.
FIG. 3 of the accompanying drawings shows the relationship between the molecular weight of toner and a temperature range in which the toner is fixable, the temperature range being a factor in toner fixing characteristics.
The hatched range shown in FIG. 3 which is surrounded by fixing starting temperatures and offset starting temperatures indicates a good toner fixing range.
The good toner fixing range is a stage prior to a flowing range in which the toner cohesion is destroyed, and is generally referred to as a rubber range. As is apparent from FIG. 3, if the width (temperature width) of the rubber range is narrow, the toner cohesive force tends to be small as is the case with a small toner molecular weight. If the width of the rubber range is narrow, therefore, the offset is liable to happen and a fixture failure is apt to occur.
It is known that the heating roller may be made of Teflon.RTM. which meets desired heat resistance and durability against a plurality of colored toners used in full-color copying.
However, even if Teflon.RTM. is used, the offset is likely to take place when toner is fixed in a narrow rubber range, and hence the desired features of Teflon.RTM. cannot be utilized in such a narrow rubber range.
In order to avoid the above problem, a heating roller in a fixing device in a full-color copying machine is made of silicone rubber. Silicone rubber is believed to be better in toner peelability than Teflon.RTM..
However, a toner fixing method using a heating roller of silicone rubber has the following disadvantages.
(1) Since silicone rubber is less durable against solvents and chemicals than Teflon.RTM., a fixing roller is contaminated by a pigment or dye of colored toner. As a result, the fixing roller has poor toner peelability and becomes short in service life. This phenomenon does not happen when only black toner is used. PA1 (2) Because silicone rubber is lower in mechanical strength than Teflon.RTM., the types of copy sheets that can be used are much more limited than would be available in a monochromatic copying process, and hence a range of usable copy sheets is limited. PA1 (3) An adhesive layer between the core of the heating roller and a silicone rubber layer around the core is susceptible to high temperatures, temperature control must be effected to give a desired level of peeling resistance to the adhesive layer on its surface held against the core. The roller surface cannot easily reach the temperature at which toner can be fixed because of the difference between heat transfer characteristics of the core and the silicone rubber layer. To solve this problem, it is proposed to position another heating roller held against the surface of the existing heating roller for keeping the surface of the silicone rubber roller at the toner fixing temperature. With this proposed arrangement, however, since the area where the heating rollers contact each other is small, heat transfer cannot well be effected therebetween, and as a consequence high-speed toner fixing cannot be performed. PA1 (4) A greater amount of toner coheres in a full-color copying process than a monochromatic copying process. Therefore, since the thickness of the toner layer is large, surface irregularities are developed on the copy sheet even if it is heated and pressed.
In the conventional fixing devices, pressing rollers have a surface layer of silicon rubber having a thickness ranging from 2.5 to 3.0 mm. Therefore, when successive copy papers are fed into the fixing device, the temperature of the pressing roller largely drops. To avoid this, it has been necessary to give a higher temperature setting to make up for such a large temperature drop.
However, such a higher temperature setting contaminates the heating roller, lowers its release capability, and shortens its service life. The temperature of the core of the pressing roller has to be controlled in view of the heat resistance of the silicone rubber layer. Since toner images are usually fixed by three rollers, their thermal responses are low, making it difficult to effect high-speed image fixing operation. Inasmuch the pressure of contact between the heating and pressing rollers per nip area has been in the range of from 2 to 3 kg/cm.sup.2, the toner layer has been thick, resulting in fixed images having surface irregularities, just like oil paintings, peculiar to full-color copies.